Electric motors are widely utilized to provide increased functionality in many of the devices utilized today. There are a variety of motor types available to suit electrical drive power needs. AC induction motors and DC motors are the most common but other motor designs such as synchronous, switched reluctance and permanent magnetic motors are used in various applications. Among the various motors, some DC motors may be used for power tools powered by batteries to provide mobility and flexibility in their use. For example, a cordless drill powered by a rechargeable battery allows a consumer to easily transport and use the drill in locations in which a corded power drill may not be useable. Since power tools need to provide a sufficient amount of torque it is desirable that the electric motor is an efficient as possible to provide the greatest amount of horsepower. With respect to battery-powered tools, efficiency in the electric motor is necessary to sustain longer battery life.
The universal motor is a rotating electric machine similar to a DC motor but designed to operate either from direct current or single-phase alternating current. Universal motors may include a rotor with a field that will oppose the field in the stator at any given moment, regardless of the polarity of the supply current changes with time. Universal motors usually run at high speeds such as 3,500 to 40,000 revolutions per minute (RPM). This results in a high power-to-weight and power-to-size ratio, making it desirable for hand-held tools, vacuum cleaners and sewing machines. However, the price of versatility is efficiency; universal motors are not as efficient (with average efficiency of 62%) as similarly-constructed AC and DC series motors.
The electrical motor utilized in the power tool significantly contributes to the overall size and weight of the power tool. Another factor of electrical motors lies in the manufacturing cost. The manufacturing cost of an electric motor contributes to a great percentage of the overall cost of the power tool. A reduction in this cost may be reflected in the cost of the power tool. Consequently, an electric motor including a universal motor and a DC motor supported by battery power that is more efficient, occupies less space, and less costly to manufacture is necessary.